Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery including: a separator including a polyolefin porous film; a porous layer containing a polyvinylidene fluoride-based resin; a positive electrode plate whose active material layer is not peeled from the positive electrode plate until the positive electrode plate is bent 130 or more times; and a negative electrode plate whose active material layer is not peeled from the negative electrode plate until the negative electrode plate is bent 1650 or more times, wherein: diethyl carbonate dropped on the polyolefin porous film diminishes at a rate of 15 sec/mg to 21 sec/mg; the diethyl carbonate has a spot diameter of not less than 20 mm 10 seconds after the diethyl carbonate was dropped on the polyolefin porous film; and the polyvinylidene fluoride-based resin contains an α-form polyvinylidene fluoride-based resin in an amount of not less than 35.0 mol %.

This Nonprovisional application claims priority under 35 U.S.C. § 119 onPatent Application No. 2017-243292 filed in Japan on Dec. 19, 2017, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries, particularly lithium-ionsecondary batteries, have a high energy density, and are therefore inwide use as batteries for a personal computer, a mobile telephone, aportable information terminal, and the like. Such nonaqueous electrolytesecondary batteries have recently been developed as on-vehiclebatteries.

For example, Patent Literature 1 discloses a nonaqueous electrolytesecondary battery including a separator for a nonaqueous electrolytesecondary battery (hereinafter referred to as a “nonaqueous electrolytesecondary battery separator”) including a polyolefin porous film thatcontrols an electrolyte retention property of the separator itself.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent No. 6153992 (Registration date: Jun. 9, 2017)

SUMMARY OF INVENTION Technical Problem

Note, however, that there is room for improvement in the aboveconventional nonaqueous electrolyte secondary battery from the viewpointof a charge capacity of the nonaqueous electrolyte secondary batterywhich has been repeatedly subjected to charge and discharge cycles.

An aspect of the present invention has an object to achieve a nonaqueouselectrolyte secondary battery that has an excellent charge capacitycharacteristic after being subjected to a charge and discharge cycle.

Solution to Problem

A nonaqueous electrolyte secondary battery in accordance with a firstaspect of the present invention includes: a nonaqueous electrolytesecondary battery separator including a polyolefin porous film; a porouslayer containing a polyvinylidene fluoride-based resin; a positiveelectrode plate whose electrode active material layer is not peeled fromthe positive electrode plate until the positive electrode plate is bent130 or more times in a folding endurance test carried out in conformitywith an MIT tester method defined in JIS P 8115 (1994), under a load of1 N, and at a bending angle of 45°; and a negative electrode plate whoseelectrode active material layer is not peeled from the negativeelectrode plate until the negative electrode plate is bent 1650 or moretimes in the folding endurance test, wherein: diethyl carbonate droppedon the polyolefin porous film diminishes at a rate of 15 sec/mg to 21sec/mg; the diethyl carbonate has a spot diameter of not less than 20 mm10 seconds after the diethyl carbonate was dropped on the polyolefinporous film; the porous layer is provided between the nonaqueouselectrolyte secondary battery separator and at least one of the positiveelectrode plate and the negative electrode plate; and the polyvinylidenefluoride-based resin contained in the porous layer contains an α-formpolyvinylidene fluoride-based resin in an amount of not less than 35.0mol % with respect to 100 mol % of a total amount of the α-formpolyvinylidene fluoride-based resin and a β-form polyvinylidenefluoride-based resin contained in the polyvinylidene fluoride-basedresin, where the amount of the α-form polyvinylidene fluoride-basedresin contained is calculated from waveform separation of (α/2) observedat around −78 ppm in a ¹⁹F-NMR spectrum obtained from the porous layer,and waveform separation of {(α/2)+β} observed at around −95 ppm in the¹⁹F-MMR spectrum obtained from the porous layer.

In a second aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the first aspect of thepresent invention, the positive electrode plate contains a transitionmetal oxide.

In a third aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the first or second aspectof the present invention, the negative electrode plate containsgraphite.

In a fourth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged in any one of the first through thirdaspects to further include: another porous layer which is providedbetween (i) the nonaqueous electrolyte secondary battery separator and(ii) at least one of the positive electrode plate and the negativeelectrode plate.

In a fifth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the fourth aspect of thepresent invention, the another porous layer contains at least one resinselected from the group consisting of a polyolefin, a(meth)acrylate-based resin, a fluorine-containing resin (excluding apolyvinylidene fluoride-based resin), a polyamide-based resin, apolyester-based resin, and a water-soluble polymer.

In a sixth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the fifth aspect of thepresent invention, the polyamide-based resin is an aramid resin.

Advantageous Effects of Invention

An aspect of the present invention makes it possible to achieve anonaqueous electrolyte secondary battery that has an excellent chargecapacity characteristic after being subjected to a charge and dischargecycle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating an MIT tester.

DESCRIPTION OF EMBODIMENTS

The following description will discuss an embodiment of the presentinvention. The present invention is, however, not limited to theembodiment below. The present invention is not limited to thearrangements described below, but may be altered in various ways by askilled person within the scope of the claims. Any embodiment based on aproper combination of technical means disclosed in different embodimentsis also encompassed in the technical scope of the present invention.Note that numerical expressions such as “A to B” herein mean “not lessthan A and not more than B” unless otherwise stated.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes: a nonaqueous electrolytesecondary battery separator including a polyolefin porous film; a porouslayer containing a polyvinylidene fluoride-based resin; a positiveelectrode plate whose electrode active material layer is not peeled fromthe positive electrode plate until the positive electrode plate is bent130 or more times in a folding endurance test carried out in conformitywith an MIT tester method defined in JIS P 8115 (1994), under a load of1 N, and at a bending angle of 45°; and a negative electrode plate whoseelectrode active material layer is not peeled from the negativeelectrode plate until the negative electrode plate is bent 1650 or moretimes in the folding endurance test, wherein: diethyl carbonate droppedon the polyolefin porous film diminishes at a rate of 15 sec/mg to 21sec/mg; the diethyl carbonate has a spot diameter of not less than 20 mm10 seconds after the diethyl carbonate was dropped on the polyolefinporous film; the porous layer is provided between the nonaqueouselectrolyte secondary battery separator and at least one of the positiveelectrode plate and the negative electrode plate; and the polyvinylidenefluoride-based resin contained in the porous layer contains an α-formpolyvinylidene fluoride-based resin in an amount of not less than 35.0mol % with respect to 100 mol % of a total amount of the α-formpolyvinylidene fluoride-based resin and a β-form polyvinylidenefluoride-based resin contained in the polyvinylidene fluoride-basedresin, where the amount of the α-form polyvinylidene fluoride-basedresin contained is calculated from waveform separation of (α/2) observedat around −78 ppm in a ¹⁹F-NMR spectrum obtained from the porous layer,and waveform separation of {(α/2)+β} observed at around −95 ppm in the¹⁹F-MMR spectrum obtained from the porous layer.

Note that, in the following description, a “polyolefin porous film” maybe referred to as a “porous film”, a “polyvinylidene fluoride-basedresin” may be referred to as a “PVDF-based resin”, a “diminution rate ofdiethyl carbonate dropped on the polyolefin porous film” may be referredto as a “diminution rate”, and a “spot diameter of the diethyl carbonate10 seconds after the diethyl carbonate was dropped on the polyolefinporous film” may be referred to as a “spot diameter”.

<Positive Electrode Plate>

The positive electrode plate of the nonaqueous electrolyte secondarybattery in accordance with an embodiment of the present invention is notlimited to any particular positive electrode plate provided that thenumber of times of bending of the positive electrode plate which numberis measured in a folding endurance test as described later falls withina specific range. For example, as a positive electrode active materiallayer, a sheet-shaped positive electrode plate is used which includes(i) a positive electrode mix containing a positive electrode activematerial, an electrically conductive agent, and a binding agent and (ii)a positive electrode current collector supporting the positive electrodemix thereon. Note that the positive electrode plate can be arranged suchthat the positive electrode current collector supports positiveelectrode mixes on respective both surfaces of the positive electrodecurrent collector or can be alternatively arranged such that thepositive electrode current collector supports the positive electrode mixon one surface of the positive electrode current collector.

Examples of the positive electrode active material include materialseach capable of being doped with and dedoped of lithium ions. Such amaterial is preferably a transition metal oxide. Specific examples ofthe transition metal oxide include a lithium complex oxide containing atleast one of transition metals such as V, Mn, Fe, Co, and Ni.

Examples of the electrically conductive agent include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and a fired product of anorganic polymer compound. These electrically conductive agents can beused in one kind or in combination of two or more kinds.

Examples of the binding agent include thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoro ethylene copolymer, athermoplastic polyimide, polyethylene, and polypropylene; acrylicresins; and styrene-butadiene rubber. The binding agent functions alsoas a thickening agent.

Examples of the positive electrode current collector include electricconductors such as Al, Ni, and stainless steel. Of these electricconductors, Al is more preferable because Al is easily processed into athin film and is inexpensive.

<Negative Electrode Plate>

The negative electrode plate of the nonaqueous electrolyte secondarybattery in accordance with an embodiment of the present invention is notlimited to any particular negative electrode plate provided that thenumber of times of bending of the negative electrode plate which numberis measured in the folding endurance test as described later fallswithin a specific range. For example, as a negative electrode activematerial layer, a sheet-shaped negative electrode plate is used whichincludes (i) a negative electrode mix containing a negative electrodeactive material and (ii) a negative electrode current collectorsupporting the negative electrode mix thereon. The sheet-shaped negativeelectrode plate preferably contains the above electrically conductiveagent and the above binding agent. Note that the negative electrodeplate can be arranged such that the negative electrode current collectorsupports negative electrode mixes on respective both surfaces of thenegative electrode current collector or can be alternatively arrangedsuch that the negative electrode current collector supports the negativeelectrode mix on one surface of the negative electrode currentcollector.

Examples of the negative electrode active material include (i) materialseach capable of being doped with and dedoped of lithium ions, (ii)lithium metals, and (iii) lithium alloys. Examples of such a materialinclude a carbonaceous material. Examples of the carbonaceous materialinclude natural graphite, artificial graphite, cokes, carbon black, andpyrolytic carbons. The sheet-shaped negative electrode plate can containan electrically conductive agent and a binding agent which are mentionedas the electrically conductive agent and the binding agent,respectively, each of which can be contained in the positive electrodeactive material layer.

Examples of the negative electrode current collector include electricconductors such as Cu, Ni, and stainless steel. Of these electricconductors, Cu is preferable because Cu is not easily alloyed withlithium particularly in a lithium-ion secondary battery and is easilyprocessed into a thin film.

<Number of Times of Bending>

The positive electrode plate and the negative electrode plate inaccordance with an embodiment of the present invention are arranged suchthat the active material layer of a corresponding one of the positiveelectrode plate and the negative electrode plate is not peeled from thepositive electrode plate or the negative electrode plate until thepositive electrode plate or the negative electrode plate is bent anumber of times which number falls within a specific range, the numberbeing measured in a folding endurance test carried out in conformitywith an MIT tester method defined in JIS P 8115 (1994). The foldingendurance test is carried out under a load of 1 N and at a bending angleof 45°. According to a nonaqueous electrolyte secondary battery,expansion and shrinkage of an active material may occur while thenonaqueous electrolyte secondary battery is being subjected to a chargeand discharge cycle. A larger number of times of bending of each of thepositive electrode plate and the negative electrode plate until theactive material layer of a corresponding one of the positive electrodeplate and the negative electrode plate is peeled from the positiveelectrode plate or the negative electrode plate, the number beingmeasured through the folding endurance test, shows that adhesivenessamong components (an active material, an electrically conductive agent,and a binder) contained in the electrode active material layer andadhesiveness between the electrode active material layer and a currentcollector are more likely to be maintained. Thus, the nonaqueouselectrolyte secondary battery can be restrained from deteriorating whilebeing subjected to a charge and discharge cycle.

In the folding endurance test, the electrode active material layer ofthe positive electrode plate is not peeled from the positive electrodeplate until the positive electrode plate is bent preferably 130 or moretimes, and more preferably 150 or more times. Meanwhile, in the foldingendurance test, the electrode active material layer of the negativeelectrode plate is not peeled from the negative electrode plate untilthe negative electrode plate is bent preferably 1650 or more times, morepreferably 1800 or more times, and still more preferably 2000 or moretimes.

FIG. 1 is a view schematically illustrating an MIT tester that is usedin the MIT tester method. In FIG. 1, an x-axis shows a horizontaldirection, and a y-axis shows a vertical direction. The followingdescription outlines the MIT tester method. The MIT tester includes aspring-loaded clamp and a bending clamp. One end and the other end of atest piece in a longer side direction of the test piece are clamped bythe spring-loaded clamp and the bending clamp, respectively, so that thetest piece is fixed. The spring-loaded clamp is connected with a weight.In the folding endurance test, a load of 1 N is applied by the weight tothe test piece. The test piece to which the load has been applied by theweight is under a tension applied in the longer side direction. In thisstate, the longer side direction of the test piece is parallel to thevertical direction. Then, the bending clamp is rotated so that the testpiece is bent. In the folding endurance test, the test piece is bent atan angle of 45° while the bending clamp is being rotated. Specifically,the test piece is bent rightward and leftward at an angle of 45°. Notethat the test piece is bent at a rate of 175 reciprocatingmovements/min.

<Method for Producing Positive Electrode Plate and Negative ElectrodePlate>

Examples of a method for producing the sheet-shaped positive electrodeplate include a method in which a positive electrode active material, anelectrically conductive agent, and a binding agent are pressure-moldedon a positive electrode current collector; and a method in which (i) apositive electrode active material, an electrically conductive agent,and a binding agent are formed into a paste with use of a suitableorganic solvent, (ii) a positive electrode current collector is coatedwith the paste, and then (iii) the paste is pressured, in a wet state orafter being dried, so that the paste is firmly fixed to the positiveelectrode current collector.

Similarly, examples of a method for producing the sheet-shaped negativeelectrode plate include a method in which a negative electrode activematerial is pressure-molded on a negative electrode current collector;and a method in which (i) a negative electrode active material is formedinto a paste with use of a suitable organic solvent, (ii) a negativeelectrode current collector is coated with the paste, and then (iii) thepaste is pressured, in a wet state or after being dried, so that thepaste is firmly fixed to the negative electrode current collector. Thepaste preferably contains the electrically conductive agent and thebinding agent.

Note here that the number of times of bending (described earlier) can becontrolled by further applying pressure to a resultant positiveelectrode plate or a resultant negative electrode plate. Specifically,the number of times of bending (described earlier) can be controlled byadjusting, for example, a time for which application of pressure iscarried out, a pressure to be applied, or a method of application ofpressure. Application of pressure is carried out for preferably 1 secondto 3600 seconds, and more preferably 1 second to 300 seconds.Application of pressure can alternatively be carried out by confiningthe positive electrode plate or the negative electrode plate. A pressurecreated by confinement is herein also referred to as a confiningpressure. The confining pressure is preferably 0.01 MPa to 10 MPa, andmore preferably 0.01 MPa to 5 MPa. Application of pressure canalternatively be carried out with use of an organic solvent while thepositive electrode plate or the negative electrode plate is made wet.This allows components contained in the electrode active material layerto further adhere to each other, and allows the electrode activematerial layer and the current collector to further adhere to eachother. Examples of the organic solvent include carbonates, ethers,esters, nitriles, amides, carbamates, and sulfur-containing compounds,and fluorine-containing organic solvents each obtained by introducing afluorine group into any of these organic solvents.

(Diminution Rate of Diethyl Carbonate Dropped on Polyolefin Porous Film)

The “diminution rate of diethyl carbonate dropped on the polyolefinporous film” herein means a speed at which the DEC that has been droppedon the polyolefin porous film evaporates, and is measured by themeasurement method below under the measurement conditions below.Measurement conditions: atmospheric pressure; room temperature(approximately 25° C.); humidity of 60% to 70%; and air velocity of notmore than 0.2 m/s; Measurement method:

(i) A square piece measuring 50 mm per side was cut out from the porousfilm and then placed on a polytetrafluoroethylene (PTFE) plate.Thereafter, the PTFE plate, on which the porous film is placed, isplaced on an analytical balance so that a zero point adjustment iscarried out.(ii) 20 mL of DEC is measured out with use of a micropipette having atip to which a pipette tip is attached.(iii) 20 μL of the DEC measured out in the step (ii) is dropped (a) froma position which is 5 mm high above the porous film placed on theanalytical balance which has been subjected to zero point adjustment inthe step (i) and (b) toward a center part of the porous film, and then ascale of the analytical balance, i.e., a weight of the DEC is measured.(iv) A time required for the weight, measured in the step (iii), of theDEC to diminish from 15 mg to 5 mg is measured, and then the time thusmeasured is divided by an amount (10 mg) by which the weight of the DEChas changed, so that the “diminution rate of diethyl carbonate droppedon the polyolefin porous film” (sec/mg) is calculated.

According to a porous film in accordance with an embodiment of thepresent invention, diethyl carbonate dropped on the porous filmdiminishes at a rate of 15 sec/mg to 21 sec/mg, preferably 16 sec/mg to20 sec/mg, and more preferably 17 sec/mg to 19 sec/mg.

If the diminution rate of diethyl carbonate dropped on the porous filmis less than 15 sec/mg, then it means that the porous film has a poorliquid retention property in a case where a nonaqueous electrolytesecondary battery separator is constituted by using a nonaqueouselectrolyte secondary battery separator including the porous film, or alaminated separator (described later) including the porous film. Thiscauses an inside of the nonaqueous electrolyte secondary battery toeasily dry out. If the diminution rate of diethyl carbonate dropped onthe porous film is more than 21 sec/mg, then it means that a fluid (anelectrolyte such as DEC or a gas generated from an electrolyte in thebattery which is being charged and discharged) moves in holes (voids) ofthe porous film at a slow speed in a case where a nonaqueous electrolytesecondary battery is constituted by using a nonaqueous electrolytesecondary battery separator including the porous film, or a laminatedseparator (described later) including the porous film. This causes theseparator to have a higher resistance to ion permeation (i.e., a lowerion permeability) as a result of (i) the battery drying out due to aninsufficient supply of an electrolyte to electrodes during batterycharge and discharge and (ii) the generated gas remaining in the voids.

<Spot Diameter of Diethyl Carbonate 10 Seconds after Diethyl Carbonatewas Dropped on Polyolefin Porous Film>

The “spot diameter of the diethyl carbonate 10 seconds after the diethylcarbonate was dropped on the polyolefin porous film” herein means adiameter of a dropped mark of the DEC remaining on the porous film after10 seconds have passed since 20 μL of DEC was dropped on the porousfilm, and is measured by the measurement method below under themeasurement conditions below.

Measurement conditions: atmospheric pressure; room temperature(approximately 25° C.); humidity of 60% to 70%; and air velocity of notmore than 0.2 m/s;

Measurement method: Steps similar to the steps (i) through (iii) of theabove method of measuring the “diminution rate of diethyl carbonatedropped on the polyolefin porous film” are carried out. Then, 20 μL ofDEC is dropped (a) from a position which is 5 mm high above the porousfilm and (b) toward a center part of the porous film. Then, after 10seconds have passed, a diameter of a dropped mark of the DEC remainingon the porous film is measured.

The porous film in accordance with an embodiment of the presentinvention is arranged such that the diethyl carbonate has a spotdiameter of not less than 20 mm, preferably not less than 21 mm, andmore preferably not less than 22 mm 10 seconds after the diethylcarbonate was dropped on the porous film. In addition, the spot diameteris preferably not more than 30 mm.

If the spot diameter of the diethyl carbonate 10 seconds after thediethyl carbonate was dropped on the porous film is less than 20 mm,then it means that the DEC thus dropped is absorbed into the voidsinside the porous film at a slow speed and thus the porous film has alow affinity for an electrolyte (such as DEC). Therefore, in a casewhere a nonaqueous electrolyte secondary battery is constituted by usinga nonaqueous electrolyte secondary battery separator including theporous film, or a laminated separator (described later) including theporous film, there is a reduction in speed at which an electrolyte suchas DEC moves in the porous film, particularly a reduction in speed atwhich the electrolyte is absorbed from an electrode mix layer into theinside of the porous film during battery charge and discharge.Meanwhile, a decrease in permeation of the electrolyte into the insideof the porous film causes the electrolyte to be retained in the porousfilm in a lower amount. This means that, in a case where battery chargeand discharge is repeated, the electrolyte can easily be depletedlocally (i) at an interface between the separator and the electrode and(ii) inside the porous base material. This results in an increase ininternal resistance of the battery, and consequently causes adeterioration in cycle characteristic of the nonaqueous electrolytesecondary battery. If the spot diameter of the diethyl carbonate 10seconds after the diethyl carbonate was dropped on the porous film ismore than 30 mm, then it means that, in a case where a nonaqueouselectrolyte secondary battery is constituted by using a nonaqueouselectrolyte secondary battery separator including the porous film, or alaminated separator (described later) including the porous film, theporous film and the electrolyte have an extremely high affinitytherebetween and thus the electrolyte can be excessively easily retainedin the porous film. This causes the electrolyte to be insufficientlysupplied to an electrode during battery charge and discharge, so thatthe battery can easily dry out.

Note that a physical property value of the porous film on which anotherlayer such as a porous layer is disposed can be measured after theanother layer is removed from a laminated body including the porous filmand the another layer. The another layer can be removed from thelaminated body by, for example, a method of dissolving a resin of theanother layer with use of a solvent such as N-methylpyrrolidone oracetone so as to remove the resin.

Note that in a case where, for example, an adhering substance(s) such asa resin powder and/or an inorganic matter is/are present on a surface ofthe porous film during measurement of the diminution rate of diethylcarbonate and the spot diameter, it is appropriately possible, beforethe measurement, to carry out a pretreatment, e.g., to (i) immerse theporous film in an organic solvent such as DEC and/or water so as toclean and remove, for example, the adhering substance(s) and then (ii)dry the solvent and the water.

<Nonaqueous Electrolyte Secondary Battery Separator>

A nonaqueous electrolyte secondary battery separator of an embodiment ofthe present invention includes a polyolefin porous film.

The porous film alone can be a nonaqueous electrolyte secondary batteryseparator. The porous film can also be a base material of a laminatedseparator for a nonaqueous electrolyte secondary battery (hereinafterreferred to as a “nonaqueous electrolyte secondary battery laminatedseparator”) (described later) in which a porous layer is disposed on theporous film. The porous film contains a polyolefin-based resin as a maincomponent and has therein many pores connected to one another. Thisallows a gas and a liquid to pass through the porous film from onesurface to the other.

The nonaqueous electrolyte secondary battery separator of an embodimentof the present invention can be arranged such that a porous layercontaining a polyvinylidene fluoride-based resin (described later) isdisposed on at least one of surfaces of the nonaqueous electrolytesecondary battery separator. In this case, a laminated body including(i) the nonaqueous electrolyte secondary battery separator and (ii) theporous layer which is disposed on at least one of the surfaces of thenonaqueous electrolyte secondary battery separator is herein referred toas a “nonaqueous electrolyte secondary battery laminated separator orlaminated separator”. The nonaqueous electrolyte secondary batteryseparator of an embodiment of the present invention can further includeother layer(s), different from the polyolefin porous film, such as anadhesive layer, a heat-resistant layer, and/or a protective layer.

(Polyolefin Porous Film)

The porous film contains polyolefin in an amount of not less than 50% byvolume, preferably not less than 90% by volume, and more preferably notless than 95% by volume, with respect to the entire porous film. Thepolyolefin more preferably contains a high molecular weight componenthaving a weight-average molecular weight of 5×10⁵ to 15×10⁶. Inparticular, the polyolefin which contains a high molecular weightcomponent having a weight-average molecular weight of not less than1,000,000 is more preferable because such polyolefin allows a nonaqueouselectrolyte secondary battery separator to have a higher strength.

Specific examples of the polyolefin, which is a thermoplastic resin,include a homopolymer and a copolymer each obtained by (co)polymerizinga monomer(s) such as ethylene, propylene, 1-butene, 4-methyl-1-pentene,and/or 1-hexene. Examples of the homopolymer include polyethylene,polypropylene, and polybutene. Examples of the copolymer include anethylene-propylene copolymer.

Among the above examples, polyethylene is more preferable. This isbecause polyethylene is capable of preventing (shutting down) a flow ofan excessively large electric current at a lower temperature. Examplesof the polyethylene include low-density polyethylene, high-densitypolyethylene, linear polyethylene (ethylene-α-olefin copolymer), andultra-high molecular weight polyethylene having a weight-averagemolecular weight of not less than 1,000,000. Of these polyethylenes,ultra-high molecular weight polyethylene having a weight-averagemolecular weight of not less than 1,000,000 is more preferable.

The porous film has a film thickness of preferably 4 μm to 40 μm, morepreferably 5 μm to 30 μm, and still more preferably 6 μm to 15 μm.

The porous film can have a weight per unit area which weight isdetermined as appropriate in view of the strength, the film thickness,the weight, and handleability of a nonaqueous electrolyte secondarybattery laminated separator including the porous film. Note, however,that the porous film has a weight per unit area of preferably 4 g/m² to20 g/m², more preferably 4 g/m² to 12 g/m², and still more preferably 5g/m² to 10 g/m², so as to allow the nonaqueous electrolyte secondarybattery which includes the nonaqueous electrolyte secondary batterylaminated separator including the porous film to have a higher weightenergy density and a higher volume energy density.

The porous film has an air permeability of preferably 30 sec/100 mL to500 sec/100 mL, and more preferably 50 sec/100 mL to 300 sec/100 mL, interms of Gurley values. The porous film which has an air permeabilityfalling within the above range makes it possible to achieve sufficiention permeability.

The porous film has a porosity of preferably 20% by volume to 80% byvolume, and more preferably 30% by volume to 75% by volume, so as to (i)retain a larger amount of electrolyte and (ii) obtain a function ofreliably preventing (shutting down) a flow of an excessively largeelectric current at a lower temperature. Further, in order to achievesufficient ion permeability and prevent particles from entering apositive electrode and/or a negative electrode of the nonaqueouselectrolyte secondary battery, the porous film has pores each having apore size of preferably not more than 0.3 μm, and more preferably notmore than 0.14 μm.

(Method for Producing Polyolefin Porous Film)

A polyolefin porous film of an embodiment of the present invention doesnot need to be produced by any particular method, and can be produced byany of various methods.

The polyolefin porous film of an embodiment of the present invention canbe produced by, for example, a method obtained by combining (i) (a) astep of extruding a polyolefin resin-based composition, in a sheet-likeshape, from a T-die at a specific T-die extrusion temperature and (b) astep of carrying out heat fixation at a specific heat fixationtemperature so as to obtain a porous film containing a polyolefin-basedresin as a main component, and (ii) a suitable step that is differentfrom the steps (a) and (b) of (i) and can be included in a common methodfor producing a polyolefin porous film (porous film). Examples of thesuitable step include a step of adding a plasticizing agent to a resinsuch as polyolefin so as to form a film and then removing theplasticizing agent with use of a suitable solvent so as to form apolyolefin porous film.

Specifically, assume, for example, that a polyolefin porous film isproduced from a polyolefin resin containing ultra-high molecular weightpolyethylene and low molecular weight polyolefin having a weight-averagemolecular weight of not more than 10,000. In this case, from theviewpoint of production costs, the polyolefin porous film is preferablyproduced by a method including the following steps:

(1) the step of obtaining a polyolefin resin composition by kneading 100parts by weight of ultra-high molecular weight polyethylene, 5 parts byweight to 200 parts by weight of low molecular weight polyolefin havinga weight-average molecular weight of not more than 10000, and 100 partsby weight to 400 parts by weight of a pore forming agent;(2) the step of using the polyolefin resin-based composition to form asheet from a T-die at a specific T-die extrusion temperature;(3) the step of removing the pore forming agent from the sheet obtainedin the step (2);(4) the step of stretching the sheet from which the pore forming agenthas been removed in the step (3); and(5) the step of subjecting the sheet stretched in the step (4) to heatfixation at a specific heat fixation temperature so as to obtain apolyolefin porous film;or(3′) the step of stretching the sheet obtained in the step (2);(4′) the step of removing the pore forming agent from the sheet whichhas been stretched in the step (3′); and(5′) the step of subjecting the sheet obtained in the step (4′) to heatfixation at a specific heat fixation temperature so as to obtain apolyolefin porous film.

Examples of the pore forming agent include an inorganic filler and aplasticizing agent.

The inorganic filler is exemplified by, but not particularly limited to,(i) an inorganic filler that can be dissolved in a water-based solventcontaining an acid, (ii) an inorganic filler that can be dissolved in awater-based solvent containing an alkali, and (iii) an inorganic fillerthat can be dissolved in a water-based solvent composed mainly of water.Examples of the inorganic filler that can be dissolved in a water-basedsolvent containing an acid include calcium carbonate, magnesiumcarbonate, barium carbonate, zinc oxide, calcium oxide, aluminumhydroxide, magnesium hydroxide, calcium hydroxide, and calcium sulfate.Of these inorganic fillers, calcium carbonate is preferable in terms ofeasiness to obtain a fine powder thereof at low cost. Examples of theinorganic filler that can be dissolved in a water-based solventcontaining an alkali include silicic acid and zinc oxide. Of theseinorganic fillers, silicic acid is preferable in terms of easiness toobtain a fine powder thereof at low cost. Examples of the inorganicfiller that can be dissolved in a water-based solvent composed mainly ofwater include calcium chloride, sodium chloride, and magnesium sulfate.

The plasticizing agent is exemplified by, but not particularly limitedto, a low molecular weight hydrocarbon such as liquid paraffin.

The T-die extrusion temperature in the step (2) is a temperature of theT-die during extrusion of the polyolefin resin composition in asheet-like shape, and is preferably not lower than 245° C. and nothigher than 280° C., and more preferably not lower than 245° C. and nothigher than 260° C.

The T-die extrusion temperature preferably falls within the above range.This is because such a T-die extrusion temperature causes a resincontained in a resultant sheet to be properly oxidized and consequentlyallows the resin to have a higher affinity for an electrolyte. Morespecifically, the T-die extrusion temperature which is increased to, forexample, not lower than 245° C. makes it possible to increase acidity ofa resin contained in the sheet, and consequently to allow the resin tohave a higher affinity for an electrolyte. This allows a resultantporous film to have an enhanced electrolyte retention property.Meanwhile, the T-die extrusion temperature which is decreased to, forexample, not higher than 280° C. makes it possible to restrain anincrease in acidity of a resin contained in the sheet, and consequentlyto cause the resin to have a lower affinity for an electrolyte. Thisallows a resultant porous film to be restrained from having anexcessively high electrolyte retention property. Therefore, the T-dieextrusion temperature which is adjusted in a proper range makes itpossible to properly adjust affinity between the resin and anelectrolyte. This allows a resultant porous film to have a moderatelyenhanced electrolyte retention property.

The heat fixation temperature in each of the steps (5) and (5′) ispreferably not lower than 100° C. and not higher than 125° C., and morepreferably not lower than 100° C. and not higher than 120° C.

The heat fixation temperature preferably falls within the above range.This is because such a heat fixation temperature allows a resultantporous film to have, inside thereof, holes (voids) whose pore size andpore channel (tortuosity) are controlled, and consequently allows anelectrolyte inside the porous film to evaporate (move) at a controlledspeed. More specifically, the heat fixation temperature which isincreased to, for example, not lower than 100° C. makes it possible toenlarge a pore size of holes in the porous film, and consequently toshorten a pore channel of the holes. This makes it possible to restrainan electrolyte inside the porous film from evaporating (moving) at ahigher speed, i.e., to restrain a resultant porous film from having anexcessively high electrolyte retention property. Meanwhile, the heatfixation temperature which is decreased to, for example, not higher than125° C. makes it possible to reduce a pore size of holes in the porousfilm, and consequently to extend a pore channel of the holes. Thisallows an electrolyte inside the porous film to evaporate (move) at alower speed, i.e., allows a resultant porous film to have an enhancedelectrolyte retention property. Therefore, the heat fixation temperaturewhich is adjusted in a proper range makes it possible to properly adjustaffinity between the resin and an electrolyte. This makes it possible torestrict, within respective specified ranges, (i) a liquid retentionproperty of a resultant porous film and (ii) a speed at which a fluidmoves in the voids.

The T-die extrusion temperature and the heat fixation temperature whichfall within the above respective ranges allow (i) an electrolyteretention property of a porous film to be produced and (ii) a speed atwhich a fluid moves in voids in the porous film to be controlled so asto fall within respective suitable ranges. This makes it possible toproduce a porous film which is arranged so that (i) diethyl carbonatedropped on the porous film diminishes at a rate of is 15 sec/mg to 21sec/mg and (ii) the diethyl carbonate has a spot diameter of not lessthan 20 mm 10 seconds after the diethyl carbonate was dropped on theporous film.

(Porous Layer)

A porous layer is provided between the nonaqueous electrolyte secondarybattery separator and at least one of the positive electrode plate andthe negative electrode plate so as to serve as a member of a nonaqueouselectrolyte secondary battery. The porous layer can be disposed on onesurface or both surfaces of the nonaqueous electrolyte secondary batteryseparator. Alternatively, the porous layer can be disposed on an activematerial layer of at least one of the positive electrode plate and thenegative electrode plate. Alternatively, the porous layer can beprovided between the nonaqueous electrolyte secondary battery separatorand at least one of the positive electrode plate and the negativeelectrode plate so as to be in contact with the nonaqueous electrolytesecondary battery separator and with the at least one of the positiveelectrode plate and the negative electrode plate. The number of porouslayer(s) which is/are provided between the nonaqueous electrolytesecondary battery separator and at least one of the positive electrodeplate and the negative electrode plate can be one, or two or more.

A porous layer is preferably an insulating porous layer containing aresin.

It is preferable that a resin which can be contained in the porous layerbe insoluble in an electrolyte of a battery and be electrochemicallystable when the battery is in normal use. In a case where the porouslayer is disposed on one surface of the porous film, the porous layer isdisposed preferably on a surface of the porous film which surface facesthe positive electrode plate of the nonaqueous electrolyte secondarybattery, and more preferably on a surface of the porous film whichsurface is in contact with the positive electrode plate.

A porous layer of an embodiment of the present invention contains aPVDF-based resin, the PVDF-based resin containing a PVDF-based resinhaving crystal form α (hereinafter, referred to as an α-form PVDF-basedresin) in an amount of not less than 35.0 mol % with respect to 100 mol% of a total amount of the α-form PVDF-based resin and a PVDF-basedresin having crystal form β (hereinafter, referred to as a β-formPVDF-based resin), the α-form PVDF-based resin and the β-form PVDF-basedresin each being contained in the PVDF-based resin.

Note here that the amount of the α-form PVDF-based resin contained iscalculated from waveform separation of (α/2) observed at around −78 ppmin a ¹⁹F-NMR spectrum obtained from the porous layer, and waveformseparation of {(α/2)+β} observed at around −95 ppm in the ¹⁹F-MMRspectrum obtained from the porous layer.

The porous layer has a structure in which many pores, connected to oneanother, are provided, so that the porous layer is a layer through whicha gas or a liquid can pass from one surface to the other. Further, in acase where the porous layer of an embodiment of the present invention isused as a member of a nonaqueous electrolyte secondary battery laminatedseparator, the porous layer can be a layer which, while serving as anoutermost layer of the nonaqueous electrolyte secondary batterylaminated separator, adheres to an electrode.

Examples of the PVDF-based resin include homopolymers of vinylidenefluoride; copolymers of vinylidene fluoride and other monomer(s)copolymerizable with vinylidene fluoride; and mixtures of the abovepolymers. Examples of a monomer polymerizable with vinylidene fluorideinclude hexafluoropropylene, tetrafluoroethylene, trifluoroethylene,trichloroethylene, and vinyl fluoride. These monomers can be used in onekind or in combination of two or more kinds. The PVDF-based resin can besynthesized through emulsion polymerization or suspensionpolymerization.

The PVDF-based resin contains vinylidene fluoride in an amount ofnormally not less than 85 mol %, preferably not less than 90 mol %, morepreferably not less than 95 mol %, and still more preferably not lessthan 98 mol %. The PVDF-based resin which contains vinylidene fluoridein an amount of not less than 85 mol % is more likely to allow theporous layer to have a mechanical strength against pressure and a heatresistance against heat during battery production.

In another aspect, the porous layer preferably contains two kinds ofPVDF-based resins (that is, a first resin and a second resin below) thatdiffer from each other in, for example, amount of hexafluoropropylenecontained. The first resin is (i) a vinylidenefluoride-hexafluoropropylene copolymer containing hexafluoropropylene inan amount of more than 0 mol % and not more than 1.5 mol % or (ii) avinylidene fluoride homopolymer.

The second resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene in an amount of more than 1.5 mol %.

The porous layer which contains the two kinds of PVDF-based resinsadheres better to an electrode as compared with the porous layer whichdoes not contain one of the two kinds of PVDF-based resins. Further, ascompared with the porous layer which does not contain one of the twokinds of PVDF-based resins, the porous layer which contains the twokinds of PVDF-based resins adheres better to another layer (e.g., aporous film layer) of the nonaqueous electrolyte secondary batteryseparator, and consequently a higher peel force is required to peel theporous layer and the another layer from each other. The first resin andthe second resin preferably have therebetween a mass ratio of 15:85 to85:15.

The PVDF-based resin preferably has a weight-average molecular weight of200,000 to 3,000,000, more preferably 200,000 to 2,000,000, and stillmore preferably 500,000 to 1,500,000. The PVDF-based resin which has aweight-average molecular weight of not less than 200,000 tends to allowthe porous layer and the electrode to adhere well to each other.Meanwhile, the PVDF-based resin which has a weight-average molecularweight of not more than 3,000,000 tends to be easily formable.

The porous layer of an embodiment of the present invention can contain aresin which is different from the PVDF-based resin and is exemplified bystyrene-butadiene copolymers; homopolymers or copolymers of vinylnitriles such as acrylonitrile and methacrylonitrile; and polyetherssuch as polyethylene oxide and polypropylene oxide.

The porous layer of an embodiment of the present invention can contain afiller. The filler can be an inorganic filler or an organic filler. Theporous layer contains the filler in an amount of preferably not lessthan 1% by mass and not more than 99% by mass, and more preferably notless than 10% by mass and not more than 98% by mass, with respect to atotal amount of the PVDF-based resin and the filler. The amount of thefiller contained in the porous layer has a lower limit that can be notless than 50% by mass, not less than 70% by mass, or not less than 90%by mass. The organic filler can be a conventionally publicly knownorganic filler, and the inorganic filler can be a conventionallypublicly known inorganic filler.

In order to achieve (i) adhesiveness of the porous layer to an electrodeand (ii) a high energy density, the porous layer of an embodiment of thepresent invention has an average thickness of preferably 0.5 μm to 10 μm(per layer), and more preferably 1 μm to 5 μm (per layer).

The porous layer preferably has a thickness of not less than 0.5 μm (perlayer). This is because the porous layer which has such a thickness (i)makes it is possible to sufficiently restrain an internal short circuitthat might occur due to, for example, breakage of the nonaqueouselectrolyte secondary battery and (ii) allows the porous layer to retainan electrolyte in an adequate amount.

Meanwhile, the porous layer which has a thickness of more than 10 μm(per layer) causes an increase in resistance to lithium ion permeationin the nonaqueous electrolyte secondary battery. Thus, the nonaqueouselectrolyte secondary battery which is repeatedly subjected to chargeand discharge cycles deteriorates in positive electrode and alsodeteriorates in rate characteristic and cycle characteristic. Further,such a porous layer makes a distance between the positive electrode andthe negative electrode greater. This causes the nonaqueous electrolytesecondary battery to have a lower internal volume efficiency.

The porous layer of an embodiment of the present invention is preferablyprovided between the nonaqueous electrolyte secondary battery separatorand the positive electrode active material layer of the positiveelectrode plate. Physical properties of the porous layer which aredescribed below at least refer to physical properties of the porouslayer which serves as a member of the nonaqueous electrolyte secondarybattery and which is provided between the nonaqueous electrolytesecondary battery separator and the positive electrode active materialof the positive electrode plate.

The porous layer can have a weight per unit area which weight isappropriately determined in view of the strength, the film thickness,the weight, and handleability of the nonaqueous electrolyte secondarybattery laminated separator. The porous layer of the nonaqueouselectrolyte secondary battery laminated separator normally has a weightper unit area of preferably 0.5 g/m² to 20 g/m², and more preferably 0.5g/m² to 10 g/m² (per layer).

The porous layer which has a weight per unit area which weight fallswithin the above numerical range allows the nonaqueous electrolytesecondary battery which includes the porous layer to have a higherweight energy density and a higher volume energy density. The porouslayer which has a weight per unit area which weight is more than theupper limit of the above range makes the nonaqueous electrolytesecondary battery heavy.

In order to achieve sufficient ion permeability, the porous layer has aporosity of preferably 20% by volume to 90% by volume, and morepreferably 30% by volume to 80% by volume. The porous layer has poreswhose pore size is preferably not more than 1.0 μm, and more preferablynot more than 0.5 μm. The pores which have a pore size falling withinthe above range allows the nonaqueous electrolyte secondary batterywhich includes the nonaqueous electrolyte secondary battery laminatedseparator including the porous layer to achieve sufficient ionpermeability.

The nonaqueous electrolyte secondary battery laminated separator has anair permeability of preferably 30 sec/100 mL to 1000 sec/100 mL, andmore preferably 50 sec/100 mL to 800 sec/100 mL, in terms of Gurleyvalues. The nonaqueous electrolyte secondary battery laminated separatorwhich has an air permeability falling within the above range allows thenonaqueous electrolyte secondary battery to achieve sufficient ionpermeability.

The air permeability which is less than the lower limit of the aboverange means that the nonaqueous electrolyte secondary battery laminatedseparator has a high porosity and thus has a coarse laminated structure.This may cause the nonaqueous electrolyte secondary battery laminatedseparator to have a lower strength and consequently to be insufficientparticularly in shape stability at a high temperature. Meanwhile, theair permeability which is more than the upper limit of the above rangemay prevent the nonaqueous electrolyte secondary battery laminatedseparator from achieving sufficient ion permeability and consequentlydegrade battery characteristics of the nonaqueous electrolyte secondarybattery.

(Crystal Forms of PVDF-Based Resin)

The PVDF-based resin contained in the porous layer which is used for anembodiment of the present invention contains an α-form PVDF-based resinin an amount of not less than 35.0 mol %, preferably not less than 37.0mol %, more preferably not less than 40.0 mol %, and still morepreferably not less than 44.0 mol %, with respect to 100 mol % of atotal amount of the α-form PVDF-based resin and a β-form PVDF-basedresin contained in the PVDF-based resin. Further, the amount of theα-form PVDF-based resin is preferably not more than 90.0 mol %. Theporous layer which contains the α-form PVDF-based resin in an amountfalling within the above range is suitably used as a member of anonaqueous electrolyte secondary battery that excels in retention of acharge capacity after being discharged at a high rate, in particular, amember of a nonaqueous electrolyte secondary battery laminated separatoror of a nonaqueous electrolyte secondary battery electrode.

The nonaqueous electrolyte secondary battery which is repeatedly chargedand discharged generates heat. The α-form PVDF-based resin contained inthe PVDF-based resin has a higher melting point than the β-formPVDF-based resin contained in the PVDF-based resin, and is less likelyto be plastically deformed by heat.

According to the porous layer of an embodiment of the present invention,in a case where the PVDF-based resin of the porous layer contains theα-form PVDF-based resin in an amount that is not less than a certainamount, it is possible to not only reduce (i) deformation of an internalstructure of the porous layer, (ii) clogging of voids in the porouslayer, and/or (iii) the like due to deformation of the PVDF-based resinwhich deformation is caused by generation of heat during repeated chargeand discharge of the nonaqueous electrolyte secondary battery, but alsoavoid uneven distribution of Li ions due to interaction between the Liions and the PVDF-based resin. As a result, it is possible to restrain adeterioration in performance of the battery.

The α-form PVDF-based resin is arranged such that the PVDF-based resinis made of a polymer containing a PVDF skeleton. The PVDF skeleton has aconformation in which there are two or more consecutive chains of asteric structure in which molecular chains include a main-chain carbonatom bonded to a fluorine atom (or a hydrogen atom) adjacent to twocarbon atoms one of which is bonded to a hydrogen atom (or a fluorineatom) having a trans position and the other (opposite) one of which isbonded to a hydrogen atom (or a fluorine atom) having a gauche position(positioned at an angle of 60°), the conformation being the followingconformation:(TGTG -type conformation)  [Math. 1]and a molecular chain having the following type:TGTG   [Math. 2]wherein respective dipole moments of C—F₂ and C—H₂ bonds each have acomponent perpendicular to the molecular chain and a component parallelto the molecular chain.

The α-form PVDF-based resin has characteristic peaks at around −95 ppmand around −78 ppm in a ¹⁹F-NMR spectrum thereof.

The β-form PVDF-based resin is arranged such that the PVDF-based resinis made of a polymer containing a PVDF skeleton. The PVDF skeleton has aconformation in which molecular chains including a main-chain carbonatom adjacent to two carbon atoms bonded to a fluorine atom and ahydrogen atom, respectively, each having a trans configuration (TT-typeconformation), that is, the fluorine atom and the hydrogen atom whichare bonded to the respective two carbon atoms are positioned oppositelyat an angle of 180° as viewed in the direction of the carbon-carbonbond.

The β-form PVDF-based resin can be arranged such that the PVDF skeletoncontained in the polymer of the PVDF-based resin has a TT-typeconformation in its entirety. The β-form PVDF-based resin canalternatively be arranged such that the PVDF skeleton partially has theTT-type conformation and has a molecular chain of the TT-typeconformation in at least four consecutive PVDF monomeric units. In anyof the above cases, (i) the carbon-carbon bond, in which the TT-typeconformation constitutes a TT-type main chain, has a planar zigzagstructure, and (ii) the respective dipole moments of the C—F₂ and C—H₂bonds each have a component perpendicular to the molecular chain.

The β-form PVDF-based resin has a characteristic peak at around −95 ppmin a ¹⁹F-NMR spectrum thereof.

(Method of Calculating Respective Percentages of α-Form PVDF-Based Resinand β-Form PVDF-Based Resin Each Contained in PVDF-Based Resin)

Respective percentages of the α-form PVDF-based resin and the β-formPVDF-based resin, each contained in the porous layer in accordance withan embodiment of the present invention, with respect to 100 mol % of thetotal amount of the α-form PVDF-based resin and the β-form PVDF-basedresin can be calculated from the a ¹⁹F-NMR spectrum obtained from theporous layer. Specifically, the percentages of the α-form PVDF-basedresin and the β-form PVDF-based resin can be calculated by, for example,the following method.

(1) From a porous layer containing a PVDF-based resin, a ¹⁹F-NMRspectrum is obtained under the following conditions.

Measurement Conditions

Measurement device: AVANCE400 manufactured by Bruker Biospin

Measurement method: Single-pulse method

Observed nucleus: ¹⁹F

Spectral bandwidth: 100 kHz

Pulse width: 3.0 s (90 pulse)

Pulse repetition time: 5.0 s

Reference material: C₆F₆ (external reference: −163.0 ppm)

Temperature: 22° C.

Sample rotation frequency: 25 kHz

(2) An integral value of a peak at around −78 ppm in the ¹⁹F-NMRspectrum obtained in (1) is calculated and is regarded as an α/2 amount.

(3) As in the case of (2), an integral value of a peak at around −95 ppmin the ¹⁹F-NMR spectrum obtained in (1) is calculated and is regarded asan {(α/2)+β} amount.

(4) A percentage of the α-form PVDF-based resin contained in the porouslayer (this percentage is hereinafter also referred to as an α ratio)with respect to 100 mol % of a total amount of the α-form PVDF-basedresin and the β-form PVDF-based resin is calculated, from the integralvalues obtained in (2) and (3), based on the following equation (1):α ratio (mol %)=[(integral value at around −78 ppm)×2/{(integral valueat around −95 ppm)+(integral value at around −78 ppm)}]×100  (1)(5) A percentage of the β-form PVDF-based resin contained in the porouslayer (this percentage is hereinafter also referred to as a β ratio)with respect to 100 mol % of the total amount of the α-form PVDF-basedresin and the β-form PVDF-based resin is calculated, from a value of theα ratio obtained in (4), based on the following equation (2):β ratio (mol %)=100(mol %)−α ratio (mol %)  (2)

(Method for Producing Porous Layer and Nonaqueous Electrolyte SecondaryBattery Laminated Separator)

The porous layer of an embodiment of the present invention and anonaqueous electrolyte secondary battery laminated separator of anembodiment of the present invention each can be produced by a methodthat is not limited to any particular method but can be any of variousmethods.

For example, a porous layer containing a PVDF-based resin and optionallya filler is formed, through one of the steps (1) through (3) below, on asurface of a porous film to serve as a base material. The steps (2) and(3) each further involve drying a deposited porous layer so as to removea solvent. Note that a coating solution that is used to produce theporous layer containing a filler and is used in each of the steps (1)through (3) is preferably a coating solution in which the filler isdispersed and in which the PVDF-based resin is dissolved.

The coating solution which is used in the method for producing theporous layer of an embodiment of the present invention can be preparednormally by (i) dissolving, in a solvent, a resin contained in theporous layer and (ii) dispersing, in a resultant solution, a fillercontained in the porous layer.

(1) Step of forming a porous layer by (i) coating a porous film with acoating solution containing a PVDF-based resin and optionally a fillerof each of which the porous layer is formed, and then (ii) drying thecoating solution so as to remove a solvent (dispersion medium) containedin the coating solution.

(2) Step of depositing a porous layer by (i) coating a surface of theporous film with the coating solution mentioned in the step (1), andthen (ii) immersing the porous film in a deposition solvent, which is apoor solvent with respect to the PVDF-based resin.

(3) Step of depositing a porous layer by (i) coating a surface of theporous film with the coating solution mentioned in the step (1), andthen (ii) making the coating solution acidic with use of a low-boilingorganic acid.

Examples of the solvent (dispersion medium) contained in the coatingsolution include N-methylpyrrolidone, N,N-dimethylacetamide,N,N-dimethylformamide, acetone, and water.

The deposition solvent is preferably isopropyl alcohol or t-butylalcohol, for example.

In the step (3), the low-boiling organic acid can be, for example,paratoluene sulfonic acid or acetic acid.

Note that the base material can be not only a porous film but also anyof a film different from the porous film, a positive electrode plate,and a negative electrode plate.

The coating solution can appropriately contain an additive(s) such as adispersing agent, a plasticizing agent, a surface active agent, and a pHadjusting agent as a component(s) different from the resin and thefiller.

Note that the base material can be not only a porous film but also anyof a film different from the porous film, a positive electrode plate,and a negative electrode plate.

The coating solution can be applied to the base material by aconventionally publicly known method that is specifically exemplified bya gravure coater method, a dip coater method, a bar coater method, and adie coater method.

(Method for Controlling Crystal Forms of PVDF-Based Resin)

Crystal forms of the PVDF-based resin contained in the porous layer ofan embodiment of the present invention can be controlled by adjustingdrying conditions under which to carry out the above-described method(e.g., the drying temperature, and the air velocity and direction duringdrying), and/or the deposition temperature at which a porous layercontaining a PVDF-based resin is deposited with use of a depositionsolvent or a low-boiling organic acid.

The drying conditions and the deposition temperature, which are adjustedso that the PVDF-based resin contains an α-form PVDF-based resin in anamount of not less than 35.0 mol % with respect to 100 mol % of a totalamount of the α-form PVDF-based resin and a β-form PVDF-based resincontained in the PVDF-based resin, can be changed as appropriate bychanging, for example, the method for producing the porous layer, a kindof solvent (dispersion medium) to be used, a kind of deposition solventto be used, and/or a kind of low-boiling organic acid to be used.

In a case where the coating solution is simply dried as in the step (1),the drying conditions can be changed as appropriate by adjusting, forexample, a kind of solvent contained in the coating solution, theconcentration of the PVDF-based resin in the coating solution, theamount of the filler (if contained), and/or the amount of the coatingsolution applied. In a case where the porous layer is formed through thestep (1), it is preferable that the drying temperature be 30° C. to 100°C., that hot air blow, during drying, perpendicularly to a porous basematerial or an electrode sheet to which the coating solution has beenapplied, and that the hot air blow at a velocity of 0.1 m/s to 40 m/s.Specifically, in a case where a coating solution to be applied containsN-methyl-2-pyrrolidone as the solvent for dissolving the PVDF-basedresin, 1.0% by mass of the PVDF-based resin, and 9.0% by mass of aluminaas the inorganic filler, the drying conditions are preferably adjustedso that (i) the drying temperature is 40° C. to 100° C., (ii) hot airblows, during drying, perpendicularly to the porous base material or theelectrode sheet to which the coating solution has been applied, and(iii) the hot air blows at a velocity of 0.4 m/s to 40 m/s.

In a case where the porous layer is formed through the step (2), it ispreferable that the deposition temperature be −25° C. to 60° C. and thatthe drying temperature be 20° C. to 100° C. Specifically, in a casewhere the porous layer is formed through the step (2) with use ofN-methylpyrrolidone as the solvent for dissolving the PVDF-based resinand isopropyl alcohol as the deposition solvent, it is preferable thatthe deposition temperature be −10° C. to 40° C. and that the dryingtemperature be 30° C. to 80° C.

(Another Porous Layer)

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can include another porous layer inaddition to (i) the porous film and (ii) the porous layer containing thePVDF-based resin. The another porous layer need only be provided between(i) the nonaqueous electrolyte secondary battery separator and (ii) atleast one of the positive electrode plate and the negative electrodeplate. The porous layer and the another porous layer can be provided inany order with respect to the nonaqueous electrolyte secondary batteryseparator. In a preferable arrangement, the porous film, the anotherporous layer, and the porous layer containing the PVDF-based resin aredisposed in this order. In other words, the another porous layer isprovided between the porous film and the porous layer containing thePVDF-based resin. In another preferable arrangement, the another porouslayer and the porous layer containing the PVDF-based resin are providedin this order on both surfaces of the porous film.

Further, the another porous layer of an embodiment of the presentinvention can contain a resin which is exemplified by polyolefins;(meth)acrylate-based resins; fluorine-containing resins (excludingpolyvinylidene fluoride-based resins); polyamide-based resins;polyimide-based resins; polyester-based resins; rubbers; resins eachhaving a melting point or a glass transition temperature of not lowerthan 180° C.; water-soluble polymers; and polycarbonate, polyacetal, andpolyether ether ketone.

Of the above resins, polyolefins, (meth)acrylate-based resins,polyamide-based resins, polyester-based resins, and water-solublepolymers are preferable.

Preferable examples of the polyolefins include polyethylene,polypropylene, polybutene, and an ethylene-propylene copolymer.

Examples of the fluorine-containing resins includepolytetrafluoroethylene, a vinylidene fluoride-hexafluoropropylenecopolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, a vinylidenefluoride-trifluoroethylene copolymer, a vinylidenefluoride-trichloroethylene copolymer, a vinylidene fluoride-vinylfluoride copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and anethylene-tetrafluoroethylene copolymer. Particular examples of thefluorine-containing resins include fluorine-containing rubber having aglass transition temperature of not higher than 23° C.

Preferable examples of the polyamide-based resins include aramid resinssuch as aromatic polyamides and wholly aromatic polyamides.

Specific examples of the aramid resins include poly(paraphenyleneterephthalamide), poly(metaphenylene isophthalamide),poly(parabenzamide), poly(metabenzamide), poly(4,4′-benzanilideterephthalamide), poly(paraphenylene-4,4′-biphenylene dicarboxylic acidamide), poly(metaphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide),poly(metaphenylene-2,6-naphthalene dicarboxylic acid amide),poly(2-chloroparaphenylene terephthalamide), a paraphenyleneterephthalamide/2,6-dichloroparaphenylene terephthalamide copolymer, anda metaphenylene terephthalamide/2,6-dichloroparaphenyleneterephthalamide copolymer. Of the above aramid resins,poly(paraphenylene terephthalamide) is more preferable.

The polyester-based resins are preferably aromatic polyesters such aspolyarylates, and liquid crystal polyesters.

Examples of the rubbers include a styrene-butadiene copolymer and ahydride thereof, a methacrylate ester copolymer, anacrylonitrile-acrylic ester copolymer, a styrene-acrylic estercopolymer, ethylene propylene rubber, and polyvinyl acetate.

Examples of the resins each having a melting point or a glass transitiontemperature of not lower than 180° C. include polyphenylene ether,polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide,polyamide imide, and polyether amide.

Examples of the water-soluble polymers include polyvinyl alcohol,polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid,polyacrylamide, and polymethacrylic acid.

Note that the above resins each of which is to be contained in theanother porous layer can be used in one kind or in combination of two ormore kinds.

The other characteristics (e.g., thickness) of the another porous layerare similar to those (of the porous layer) described above, except thatthe porous layer contains the PVDF-based resin.

<Nonaqueous Electrolyte>

A nonaqueous electrolyte that can be contained in the nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention is not limited to any particular nonaqueouselectrolyte provided that the nonaqueous electrolyte is a nonaqueouselectrolyte for common use in a nonaqueous electrolyte secondarybattery. The nonaqueous electrolyte can be, for example, a nonaqueouselectrolyte containing an organic solvent and a lithium salt dissolvedin the organic solvent. Examples of the lithium salt include LiClO₄,LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃,Li₂B₁₀Cl₁₀, lower aliphatic carboxylic acid lithium salt, and LiAlCl₄.These lithium salts can be used in one kind or in combination of two ormore kinds.

Examples of the organic solvent which is contained in the nonaqueouselectrolyte include carbonates, ethers, esters, nitriles, amides,carbamates, and sulfur-containing compounds, and fluorine-containingorganic solvents each obtained by introducing a fluorine group into anyof these organic solvents. These organic solvents can be used in onekind or in combination of two or more kinds.

<Method for Producing Nonaqueous Electrolyte Secondary Battery>

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can be produced by, for example, (i)forming a member for a nonaqueous electrolyte secondary battery(hereinafter referred to as a nonaqueous electrolyte secondary batterymember) by providing the positive electrode plate, the nonaqueouselectrolyte secondary battery separator, and the negative electrodeplate in this order, (ii) placing the nonaqueous electrolyte secondarybattery member in a container which is to serve as a housing of thenonaqueous electrolyte secondary battery, (iii) filling the containerwith a nonaqueous electrolyte, and then (iv) hermetically sealing thecontainer under reduced pressure.

As described earlier, a nonaqueous electrolyte secondary battery inaccordance with an embodiment of the present invention includes: anonaqueous electrolyte secondary battery separator including apolyolefin porous film; a porous layer; a positive electrode plate; anda negative electrode plate. In particular, the nonaqueous electrolytesecondary battery in accordance with an embodiment of the presentinvention satisfies the following requirements (i) through (iv):

(i) the requirement that a polyvinylidene fluoride-based resin containedin the porous layer contains an α-form polyvinylidene fluoride-basedresin in an amount of not less than 35.0 mol % with respect to 100 mol %of a total amount of the α-form polyvinylidene fluoride-based resin anda β-form polyvinylidene fluoride-based resin contained in the porouslayer;(ii) the requirement that an electrode active material layer of thepositive electrode plate is not peeled from the positive electrode plateuntil the positive electrode plate is bent 130 or more times;(iii) the requirement that an electrode active material layer of thenegative electrode plate is not peeled from the negative electrode plateuntil the negative electrode plate is bent 1650 or more times; and(iv) the requirement that diethyl carbonate dropped on the polyolefinporous film diminishes at a rate of is 15 sec/mg to 21 sec/mg, and thediethyl carbonate has a spot diameter of not less than 20 mm 10 secondsafter the diethyl carbonate was dropped on the polyolefin porous film.

According to the nonaqueous electrolyte secondary battery in accordancewith an embodiment of the present invention which nonaqueous electrolytesecondary battery satisfies the requirement (i), it is possible torestrain the PVDF-based resin from being plastically deformed at a hightemperature, and to prevent structural deformation of the porous layerand clogging of voids in the porous layer after the nonaqueouselectrolyte secondary battery is subjected to a charge and dischargecycle. Furthermore, satisfaction of the requirements (ii) and (iii)makes it easy for an entire electrode to isotropically follow expansionand shrinkage of an active material. This makes it easier to maintainadhesiveness between components contained in the electrode activematerial layer, and adhesiveness between the electrode active materiallayer and a current collector. Moreover, satisfaction of the requirement(iv) makes it possible to control a nonaqueous electrolyte retentionproperty of the polyolefin porous film and a fluid moving speed, whichis a speed at which a fluid moves in voids in the porous film, so thatthe nonaqueous electrolyte retention property and the fluid moving speedfall within respective suitable ranges.

Thus, according to the nonaqueous electrolyte secondary battery whichsatisfies the above requirements (i) through (iv), (a) the porous layerhas favorable ion permeability because the porous layer is favorablystructurally stable after the nonaqueous electrolyte secondary batteryis subjected to a charge and discharge cycle, (b) the nonaqueouselectrolyte favorably moves in the polyolefin porous film because theabove fluid moving speed is controlled so as to fall within a suitablerange, and (c) a deterioration in nonaqueous electrolyte secondarybattery which is being subjected to a charge and discharge cycle isrestrained because the above-described adhesiveness is easilymaintained. Therefore, the battery is considered to have a higher chargecapacity even after being subjected to a charge and discharge cycle (forexample, after 100 cycles have passed).

The present invention is not limited to the embodiments, but can bealtered by a skilled person in the art within the scope of the claims.The present invention also encompasses, in its technical scope, anyembodiment derived by combining technical means disclosed in differingembodiments.

EXAMPLES

The following description will more specifically discuss the presentinvention with reference to Examples and Comparative Examples. Note,however, that the present invention is not limited to the Examples.

[Measurement Method]

Measurements were carried out in Examples and Comparative Examples bythe method below.

(1) Diminution Rate of Diethyl Carbonate Dropped on Polyolefin PorousFilm

A square piece which measured 50 mm per side and was to be subjected tomeasurement was cut out from each of porous films obtained in Examplesand Comparative Examples, and then was placed on apolytetrafluoroethylene (PTFE) plate at an atmospheric pressure, at aroom temperature (approximately 25° C.), at a humidity (approximately60% to 70%), and at an air velocity of not more than 0.2 m/s. Then, thepolytetrafluoroethylene (PTFE) plate on which the square piece had beenplaced was placed on an analytical balance (manufactured by ShimadzuCorporation, model name: AUW220) so as to be subjected to zero pointadjustment. Then, diethyl carbonate (DEC) was measured out with use of amicropipette (manufactured by Eppendorf, model name: Reference, designedfor 20 μL) having a tip to which a pipette tip (manufactured byEppendorf, product name: Standard, yellow tip designed for 0.5 μL to 20μL) was attached. After zero point adjustment was carried out, 20 μL ofthe DEC thus measured out was dropped, from a position 5 mm high, on acenter part of a porous film, and then an amount of change in weight ofthe DEC was measured. Specifically, a time required for the weight ofthe DEC to diminish from 15 mg to 5 mg (hereinafter referred to also asan “evaporation time”) was measured. Then, the “evaporation time” thusmeasured was divided by an amount (10 mg) by which the weight of the DEChad changed, so that an obtained value was regarded as a measured valueof the “diminution rate”.

(2) Spot Diameter of Diethyl Carbonate 10 Seconds after DiethylCarbonate was Dropped on Polyolefin Porous Film

Under conditions similar to those for the measurement of the “diminutionrate” and by a method similar to that for the measurement of the“diminution rate”, 20 μL of DEC, which had been measured out, wasdropped, from a position 5 mm high, on a center part of each of theporous films produced in Examples and Comparative Examples. After 10seconds passed, a diameter of a dropped mark of the DEC remaining on theporous film was measured. Then, a measured value was regarded as ameasured value of the “spot diameter”.

The “diminution rate” and the “spot diameter” of each of the porousfilms produced in Examples and Comparative Examples were each measuredthree times in total. A value of the “diminution rate” was calculated byaveraging three values obtained through the measurement of the“diminution rate”, and a value of the “spot diameter” was calculated byaveraging three values obtained through the measurement of the “spotdiameter”.

(3) α Ratio Calculation Method

A laminated separator piece having a size of approximately 2 cm×5 cm wascut out from a laminated separator produced in each of Examples andComparative Examples below. In accordance with the steps (1) through (4)of the above (Method of calculating respective percentages of α-formPVDF-based resin and β-form PVDF-based resin each contained inPVDF-based resin), a percentage (a ratio) of an α-form PVDF-based resincontained in a PVDF-based resin contained in the laminated separatorpiece thus cut out was measured.

(4) Folding Endurance Test

A test piece having a length of 105 mm and a width of 15 mm was cut outfrom a positive electrode plate or a negative electrode plate producedin each of Examples and Comparative Examples below. A folding endurancetest was carried out with use of the test piece in conformity with theMIT tester method.

The folding endurance test was carried out with use of an MIT-typefolding endurance tester (manufactured by Yasuda Seiki Seisakusho,Ltd.), in conformity with the MIT tester method defined in JIS P 8115(1994), and under the condition that a load was 1 N, a bent part had aradius of 0.38 mm, and the test piece was bent at a rate of 175reciprocating movements/min. The test piece was bent 45° from side toside while an end of the test piece was fixed.

According to the above method, the number of times of bending of thepositive electrode plate or the negative electrode plate until anelectrode active material layer of a corresponding one of the positiveelectrode plate and the negative electrode plate is peeled from thepositive electrode plate or the negative electrode plate was measured.The number of times of bending herein refers to the number of times ofreciprocating bending which number is displayed on a counter of theMIT-type folding endurance tester.

(5) Charge Capacity in 100th Charge and Discharge Cycle

A charge capacity obtained in the 100th charge and discharge cycle in acase where a nonaqueous electrolyte secondary battery produced in eachof Examples and Comparative Examples had been subjected to 100 chargeand discharge cycles repeated was measured by a method including thefollowing steps (A) and (B).

(A) Initial Charge and Discharge Test

A new nonaqueous electrolyte secondary battery which had been producedin each of Examples and Comparative Examples and which had not beensubjected to any charge and discharge cycle was subjected to 4 initialcharge and discharge cycles at 25° C. Each of the 4 initial charge anddischarge cycles was carried out under the condition that a voltageranged from 2.7 V to 4.1 V, CC-CV charge was carried out at a chargecurrent value of 0.2 C (terminal current condition: 0.02 C), and CCdischarge was carried out at a discharge current value of 0.2 C. Notethat 1 C is defined as a value of an electric current at which a ratedcapacity based on a discharge capacity at 1 hour rate is discharged in 1hour. Same applies to the following description. Note that the CC-CVcharge is a charging method in which a battery is charged at a setconstant electric current, and after a given voltage is reached, thegiven voltage is maintained while the electric current is reduced. Notealso that the CC discharge is a discharging method in which a battery isdischarged at a set constant electric current until a given voltage isreached. Same applies to the following description.

(B) Cycle Test

The nonaqueous electrolyte secondary battery which had been subjected tothe initial charge and discharge test was subjected to 100 charge anddischarge cycles at 55° C. Each of the charge and discharge 100 cycleswas carried out under the condition that a voltage ranged from 2.7 V to4.2 V, CC-CV charge was carried out at a charge current value of 1 C(terminal current condition: 0.02 C), and CC discharge was carried outat a discharge current value of 10 C.

Table 1 shows a value obtained by dividing, by a mass of a positiveelectrode active material, a charge capacity obtained in the 100thcharge and discharge cycle (mAh) in a case where the nonaqueouselectrolyte secondary battery had been subjected to 100 charge anddischarge cycles repeated.

Example 1

[Production of Nonaqueous Electrolyte Secondary Battery LaminatedSeparator]

Ultra-high molecular weight polyethylene powder (GUR4032, manufacturedby Ticona Corporation) having a weight-average molecular weight of4,970,000 and polyethylene wax (FNP-0115, manufactured by Nippon SeiroCo., Ltd.) having a weight-average molecular weight of 1,000 were mixedso as to prepare a mixture containing the ultra-high molecular weightpolyethylene powder in an amount of 70% by weight and the polyethylenewax in an amount of 30% by weight. Assuming that the ultra-highmolecular weight polyethylene powder and the polyethylene wax of themixture had 100 parts by weight in total, to the 100 parts by weight ofthe mixture, 0.4 parts by weight of an antioxidant (Irg1010,manufactured by Ciba Specialty Chemicals Inc.), 0.1 parts by weight ofan antioxidant (P168, manufactured by Ciba Specialty Chemicals Inc.),and 1.3 parts by weight of sodium stearate were added, and then calciumcarbonate (manufactured by Maruo Calcium Co., Ltd.) having an averageparticle diameter of 0.1 μm was further added so as to account for 36%by volume of a total volume of a resultant mixture. Then, the resultantmixture was mixed as it was, that is, in a form of powder, in a Henschelmixer, so that a mixture 1 was obtained.

Then, the mixture 1 was melted and kneaded in a twin screw kneadingextruder, so that a polyolefin resin composition 1 was obtained. Thepolyolefin resin composition 1 was extruded, in a form of a sheet, froma T-die whose temperature was set at 250° C., and the sheet was rolledwith use of a pair of rolls each having a surface temperature of 150°C., so that a rolled sheet 1 was prepared. Subsequently, the rolledsheet 1 was immersed in an aqueous hydrochloric acid solution(containing 4 mol/L of hydrochloric acid and 0.5% by weight of anonionic surface active agent) so that the calcium carbonate was removedfrom the rolled sheet 1. Then, the rolled sheet was stretched at astretch ratio of 6.2 times. Furthermore, the rolled sheet was subjectedto heat fixation at 120° C., so that a porous film 1 was obtained.

An N-methyl-2-pyrrolidone (hereinafter also referred to as “NMP”)solution (manufactured by Kureha Corporation; product name: “L #9305”,weight-average molecular weight: 1,000,000) containing a PVDF-basedresin (polyvinylidene fluoride-hexafluoropropylene copolymer) wasprepared as a coating solution. The coating solution was applied to theporous film 1 by a doctor blade method so that the PVDF-based resincontained in the coating solution which had been applied to the porousfilm 1 weighed 6.0 g per square meter of the porous film 1.

A resultant coated product was immersed in 2-propanol while a coatedfilm thereof was wet with a solvent contained in the coating solution,and then was left to stand still at −10° C. for 5 minutes, so that alaminated porous film 1 was obtained. The laminated porous film 1obtained was further immersed in other 2-propanol while being wet withthe above immersion solvent, and then was left to stand still at 25° C.for 5 minutes, so that a laminated porous film 1 a was obtained. Thelaminated porous film 1 a obtained was dried at 30° C. for 5 minutes, sothat a laminated separator 1 in which a porous layer was disposed on thelaminated porous film 1 a was produced. Table 1 shows results ofevaluation of the laminated separator 1 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

(Positive Electrode Plate)

A positive electrode plate was obtained in which a positive electrodemix (a mixture of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electricallyconductive agent, and PVDF (at a weight ratio of 92:5:3)) was disposedon one surface of a positive electrode current collector (aluminumfoil). To the positive electrode plate, a confining pressure (0.7 MPa)was applied at a room temperature for 30 seconds.

The positive electrode plate was cut off so that a cut piece of thepositive electrode plate had (i) a first part, on which a positiveelectrode active material layer was disposed, had a size of 45 mm×30 mmand (ii) a second part, on which no positive electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A positive electrode plate 1 was thus obtained.

(Negative Electrode Plate)

A negative electrode plate was obtained in which a negative electrodemix (a mixture of natural graphite, a styrene-1,3-butadiene copolymer,and sodium carboxymethylcellulose (at a weight ratio of 98:1:1)) wasdisposed on one surface of a negative electrode current collector(copper foil). To the negative electrode plate, a confining pressure(0.7 MPa) was applied at a room temperature for 30 seconds.

The negative electrode plate was cut off so that a cut piece of thenegative electrode plate had (i) a first part, on which a negativeelectrode active material layer was disposed, had a size of 50 mm×35 mmand (ii) a second part, on which no negative electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A negative electrode plate 1 was thus obtained.

(Assembly of Nonaqueous Electrolyte Secondary Battery)

The positive electrode plate 1, the negative electrode plate 1, and thelaminated separator 1 were used to produce a nonaqueous electrolytesecondary battery by the following method.

Specifically, (i) the positive electrode plate 1, (ii) the laminatedseparator 1 including a porous layer provided so as to face the positiveelectrode plate 1, and (iii) the negative electrode plate 1 weredisposed on top of each other (provided) in this order in a laminatepouch, so that a nonaqueous electrolyte secondary battery member 1 wasobtained. In this case, the positive electrode plate 1 and the negativeelectrode plate 1 were provided so that a whole of a main surface of thepositive electrode active material layer of the positive electrode plate1 was included in a range of a main surface (overlapped the mainsurface) of the negative electrode active material layer of the negativeelectrode plate 1.

Subsequently, the nonaqueous electrolyte secondary battery member 1 wasput into a bag which had been made, in advance, of a laminate of analuminum layer and a heat seal layer. Further, 0.25 mL of nonaqueouselectrolyte was put into the bag. The above nonaqueous electrolyte wasprepared by dissolving LiPF₆ in a mixed solvent of ethylene carbonate,ethyl methyl carbonate, and diethyl carbonate at a ratio (volume ratio)of 3:5:2 so that the LiPF₆ was contained at 1 mol/L. Then, the bag washeat-sealed while the pressure inside the bag was reduced, so that anonaqueous electrolyte secondary battery 1 was produced.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 1produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 2

[Production of Nonaqueous Electrolyte Secondary Battery LaminatedSeparator]

Example 2 produced a porous film 2 as in the case of Example 1 exceptthat Example 2 changed the heat fixation temperature to 110° C.

Example 2 applied a coating solution to the porous film 2 as in the caseof Example 1. A resultant coated product was immersed in 2-propanolwhile a coated film thereof was wet with a solvent contained in thecoating solution, and then was left to stand still at 25° C. for 5minutes, so that a laminated porous film 2 was obtained. The laminatedporous film 2 obtained was further immersed in other 2-propanol whilebeing wet with the above immersion solvent, and then was left to standstill at 25° C. for 5 minutes, so that a laminated porous film 2 a wasobtained. The laminated porous film 2 a obtained was dried at 65° C. for5 minutes, so that a laminated separator 2 in which a porous layer wasdisposed on the laminated porous film 2 a was produced. Table 1 showsresults of evaluation of the laminated separator 2 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 2 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 2 used the laminated separator 2instead of the laminated separator 1. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 2.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 2produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 3

[Production of Nonaqueous Electrolyte Secondary Battery LaminatedSeparator]

Example 3 obtained a porous film 3 as in the case of Example 1 exceptthat Example 3 (i) used (a) ultra-high molecular weight polyethylenepowder (GUR4032, manufactured by Ticona Corporation) in an amount of71.5% by weight and (b) polyethylene wax (FNP-0115, manufactured byNippon Seiro Co., Ltd.), having a weight-average molecular weight of1000, in an amount of 28.5% by weight, (ii) added calcium carbonate(manufactured by Maruo Calcium Co., Ltd.), having an average particlediameter of 0.1 μm, so that the calcium carbonate accounted for 37% byvolume of a total volume of a resultant mixture, (iii) set a stretchratio at 7.0 times, and (iv) set the heat fixation temperature at 123°C.

Example 3 applied a coating solution to the porous film 3 as in the caseof Example 1. A resultant coated product was immersed in 2-propanolwhile a coated film thereof was wet with a solvent contained in thecoating solution, and then was left to stand still at −5° C. for 5minutes, so that a laminated porous film 3 was obtained. The laminatedporous film 3 obtained was further immersed in other 2-propanol whilebeing wet with the above immersion solvent, and then was left to standstill at 25° C. for 5 minutes, so that a laminated porous film 3 a wasobtained. The laminated porous film 3 a obtained was dried at 30° C. for5 minutes, so that a laminated separator 3 in which a porous layer wasdisposed on the laminated porous film 3 a was produced. Table 1 showsresults of evaluation of the laminated separator 3 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 3 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 3 used the laminated separator 3instead of the laminated separator 1. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 3.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 3produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 4

(Positive Electrode Plate)

A positive electrode plate was obtained in which a positive electrodemix (a mixture of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electricallyconductive agent, and PVDF (at a weight ratio of 92:5:3)) was disposedon one surface of a positive electrode current collector (aluminumfoil). To the positive electrode plate, which was made wet with diethylcarbonate, a confining pressure (0.7 MPa) was applied at a roomtemperature for 30 seconds.

The positive electrode plate was cut off so that a cut piece of thepositive electrode plate had (i) a first part, on which a positiveelectrode active material layer was disposed, had a size of 45 mm×30 mmand (ii) a second part, on which no positive electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A positive electrode plate 2 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 4 used the negative electrode plate 1 as a negative electrodeplate. Furthermore, Example 4 produced a nonaqueous electrolytesecondary battery as in the case of Example 1 except that Example 4 (i)used the laminated separator 3 instead of the laminated separator 1 and(ii) used the positive electrode plate 2 instead of the positiveelectrode plate 1. The nonaqueous electrolyte secondary battery thusproduced was designated as a nonaqueous electrolyte secondary battery 4.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 4produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 5

(Positive Electrode Plate)

A positive electrode plate was obtained in which a positive electrodemix (a mixture of LiCoO₂, an electrically conductive agent, and PVDF (ata weight ratio of 100:5:3)) was disposed on one surface of a positiveelectrode current collector (aluminum foil). To the positive electrodeplate, which was made wet with diethyl carbonate, a confining pressure(0.7 MPa) was applied at a room temperature for 30 seconds.

The positive electrode plate was cut off so that a cut piece of thepositive electrode plate had (i) a first part, on which a positiveelectrode active material layer was disposed, had a size of 45 mm×30 mmand (ii) a second part, on which no positive electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A positive electrode plate 3 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 5 used the negative electrode plate 1 as a negative electrodeplate. Furthermore, Example 5 produced a nonaqueous electrolytesecondary battery as in the case of Example 1 except that Example 5 (i)used the laminated separator 3 instead of the laminated separator 1 and(ii) used the positive electrode plate 3 instead of the positiveelectrode plate 1. The nonaqueous electrolyte secondary battery thusproduced was designated as a nonaqueous electrolyte secondary battery 5.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 5produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 6

(Negative Electrode Plate)

A negative electrode plate was obtained in which a negative electrodemix (a mixture of natural graphite, a styrene-1,3-butadiene copolymer,and sodium carboxymethylcellulose (at a weight ratio of 98:1:1)) wasdisposed on one surface of a negative electrode current collector(copper foil). To the negative electrode plate, which was made wet withdiethyl carbonate, a confining pressure (0.7 MPa) was applied at a roomtemperature for 30 seconds.

The negative electrode plate was cut off so that a cut piece of thenegative electrode plate had (i) a first part, on which a negativeelectrode active material layer was disposed, had a size of 50 mm×35 mmand (ii) a second part, on which no negative electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A negative electrode plate 2 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 6 used the positive electrode plate 1 as a positive electrodeplate. Furthermore, Example 6 produced a nonaqueous electrolytesecondary battery as in the case of Example 1 except that Example 6 (i)used the laminated separator 3 instead of the laminated separator 1 and(ii) used the negative electrode plate 2 instead of the negativeelectrode plate 1. The nonaqueous electrolyte secondary battery thusproduced was designated as a nonaqueous electrolyte secondary battery 6.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 6produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 7

(Negative Electrode Plate)

A negative electrode plate was obtained in which a negative electrodemix (a mixture of artificial spherulite graphite, an electricallyconductive agent, and PVDF (at a weight ratio of 85:15:7.5) was disposedon one surface of a negative electrode current collector (copper foil).To the negative electrode plate, which was made wet with diethylcarbonate, a confining pressure (0.7 MPa) was applied at a roomtemperature for 30 seconds.

The negative electrode plate was cut off so that a cut piece of thenegative electrode plate had (i) a first part, on which a negativeelectrode active material layer was disposed, had a size of 50 mm×35 mmand (ii) a second part, on which no negative electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A negative electrode plate 3 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 7 used the positive electrode plate 1 as a positive electrodeplate. Furthermore, Example 7 produced a nonaqueous electrolytesecondary battery as in the case of Example 1 except that Example 7 (i)used the laminated separator 3 instead of the laminated separator 1 and(ii) used the negative electrode plate 3 instead of the negativeelectrode plate 1. The nonaqueous electrolyte secondary battery thusproduced was designated as a nonaqueous electrolyte secondary battery 7.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 7produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Example 8

[Production of Porous Layer and Laminated Separator]

A PVDF-based resin (manufactured by Arkema Inc.; product name “Kynar(Registered Trademark) LBG”, having a weight-average molecular weight of590,000) was dissolved, by being stirred at 65° C. over 30 minutes, inN-methyl-2-pyrrolidone so that a solid content in a resultant solutionwas 10% by mass. The resultant solution was used as a binder solution.As a filler, alumina fine particles (manufactured by Sumitomo ChemicalCo., Ltd.; product name “AKP3000”, containing 5 ppm of silicon) wereused. The alumina fine particles, the binder solution, and a solvent(N-methyl-2-pyrrolidone) were mixed together at the following ratio.That is, the alumina fine particles, the binder solution, and thesolvent were mixed together so that (i) a resultant mixed solutioncontained 10 parts by weight of the PVDF-based resin with respect to 90parts by weight of the alumina fine particles and (ii) a concentrationof a solid content (alumina fine particles+PVDF-based resin) in themixed solution was 10% by weight. A dispersion solution was thusobtained.

A coating solution was applied to the porous film 3, which had beenproduced in Example 3, by a doctor blade method so that the PVDF-basedresin contained in the coating solution which had been applied to theporous film 3 weighed 6.0 g per square meter of the porous film 3. Alaminated porous film 4 was thus produced. The laminated porous film 4was dried at 65° C. for 5 minutes, so that a laminated separator 4 wasproduced. The laminated porous film 4 was dried by hot air blownperpendicularly to the laminated porous film 4 at an air velocity of 0.5m/s. Table 1 shows results of evaluation of the laminated separator 4produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 8 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 8 used the laminated separator 4instead of the laminated separator 1. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 8.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 8produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Comparative Example 1

[Production of Nonaqueous Electrolyte Secondary Battery Separator]

A coated product obtained as in the case of the obtainment of the coatedproduct in Example 3 was immersed in 2-propanol while a coated filmthereof was wet with a solvent contained in a coating solution, and thenwas left to stand still at −78° C. for 5 minutes, so that a laminatedporous film 5 was obtained. The laminated porous film 5 obtained wasfurther immersed in other 2-propanol while being wet with the aboveimmersion solvent, and then was left to stand still at 25° C. for 5minutes, so that a laminated porous film 5 a was obtained. The laminatedporous film 5 a obtained was dried at 30° C. for 5 minutes, so that alaminated separator 5 was produced. Table 1 shows the results ofevaluation of the laminated separator 5 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Comparative Example 1 produced a nonaqueous electrolyte secondarybattery as in the case of Example 1 except that Comparative Example 1used the laminated separator 5 instead of the laminated separator 1. Thenonaqueous electrolyte secondary battery thus produced was designated asa nonaqueous electrolyte secondary battery 9.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 9produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Comparative Example 2

(Positive Electrode Plate)

A positive electrode plate was obtained in which a positive electrodemix (a mixture of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electricallyconductive agent, and PVDF (at a weight ratio of 92:5:3)) was disposedon one surface of a positive electrode current collector (aluminumfoil).

The positive electrode plate was cut off so that a cut piece of thepositive electrode plate had (i) a first part, on which a positiveelectrode active material layer was disposed, had a size of 45 mm×30 mmand (ii) a second part, on which no positive electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A positive electrode plate 4 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Comparative Example 2 used the negative electrode plate 1 as a negativeelectrode plate. Furthermore, Comparative Example 2 produced anonaqueous electrolyte secondary battery as in the case of Example 1except that Comparative Example 2 (i) used the laminated separator 3instead of the laminated separator 1 and (ii) used the positiveelectrode plate 4 instead of the positive electrode plate 1. Thenonaqueous electrolyte secondary battery thus produced was designated asa nonaqueous electrolyte secondary battery 10.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 10produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

Comparative Example 3

(Negative Electrode Plate)

A negative electrode plate was obtained in which a negative electrodemix (a mixture of natural graphite, a styrene-1,3-butadiene copolymer,and sodium carboxymethylcellulose (at a weight ratio of 98:1:1)) wasdisposed on one surface of a negative electrode current collector(copper foil).

The negative electrode plate was cut off so that a cut piece of thenegative electrode plate had (i) a first part, on which a negativeelectrode active material layer was disposed, had a size of 50 mm×35 mmand (ii) a second part, on which no negative electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A negative electrode plate 4 was thus obtained.

[Production of Nonaqueous Electrolyte Secondary Battery]

Comparative Example 3 used the positive electrode plate 1 as a positiveelectrode plate. Furthermore, Comparative Example 3 produced anonaqueous electrolyte secondary battery as in the case of Example 1except that Comparative Example 3 (i) used the laminated separator 3instead of the laminated separator 1 and (ii) used the negativeelectrode plate 4 instead of the negative electrode plate 1. Thenonaqueous electrolyte secondary battery thus produced was designated asa nonaqueous electrolyte secondary battery 11.

Thereafter, a charge capacity obtained in the 100th charge and dischargecycle in a case where the nonaqueous electrolyte secondary battery 11produced by the above method had been subjected to 100 charge anddischarge cycles repeated was measured. Table 1 shows results of themeasurement.

TABLE 1 Laminated separator Porous film Porous layer Diminution rateSpot diameter PVDF α ratio (sec/mg) (mm) (mol %) Example 1 15.1 23 35.3Example 2 20.4 21 80.8 Example 3 17.8 21 44.4 Example 4 17.8 21 44.4Example 5 17.8 21 44.4 Example 6 17.8 21 44.4 Example 7 17.8 21 44.4Example 8 17.8 21 64.3 Comparative 17.8 21 34.6 Example 1 Comparative17.8 21 44.4 Example 2 Comparative 17.8 21 44.4 Example 3 BatteryElectrode Charging Positive electrode Negative electrode characteristicNumber of times of Number of times of Charge bending carried out bendingcarried out capacity until electrode until electrode obtained activematerial active material in 100th layer is peeled layer is peeled cycle(mAh/g) Example 1 164 1732 128 Example 2 164 1732 116 Example 3 164 173299 Example 4 177 1732 97 Example 5 210 1732 107 Example 6 164 1858 112Example 7 164 2270 121 Example 8 164 1732 114 Comparative 164 1732 60Example 1 Comparative 126 1732 80 Example 2 Comparative 164 1633 76Example 3

Table 1 shows that, as compared with the nonaqueous electrolytesecondary battery produced in each of Comparative Examples 1 to 3, thenonaqueous electrolyte secondary battery produced in each of Examples 1to 8 had a higher charge capacity obtained in the 100th charge anddischarge cycle in a case where the nonaqueous electrolyte secondarybattery had been subjected to 100 charge and discharge cycles repeated.

This reveals that, in a case where a nonaqueous electrolyte secondarybattery satisfies the following four requirements: (i) the requirementthat a polyvinylidene fluoride-based resin contained in a porous layercontains an α-form polyvinylidene fluoride-based resin in an amount ofnot less than 35.0 mol % with respect to 100 mol % of a total amount ofthe α-form polyvinylidene fluoride-based resin and a β-formpolyvinylidene fluoride-based resin contained in the porous layer; (ii)the requirement that an electrode active material layer of a positiveelectrode plate is not peeled from the positive electrode plate untilthe positive electrode plate is bent 130 or more times; (iii) therequirement that an electrode active material layer of a negativeelectrode plate is not peeled from the negative electrode plate untilthe negative electrode plate is bent 1650 or more times; and (iv) therequirement that diethyl carbonate dropped on a polyolefin porous filmdiminishes at a rate of is 15 sec/mg to 21 sec/mg, and the diethylcarbonate has a spot diameter of not less than 20 mm 10 seconds afterthe diethyl carbonate was dropped on the polyolefin porous film, thenonaqueous electrolyte secondary battery can have a better chargecapacity characteristic after being subjected to a charge and dischargecycle.

Reference Example 1

(Positive Electrode Plate)

A positive electrode plate was obtained in which a positive electrodemix (a mixture of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electricallyconductive agent, and PVDF (at a weight ratio of 92:5:3)) was disposedon one surface of a positive electrode current collector (aluminumfoil). To the positive electrode plate, a pressure of 40 MPa was appliedat a room temperature with use of a roll pressing machine. A positiveelectrode plate A was thus obtained.

(Negative Electrode Plate)

A negative electrode plate was obtained in which a negative electrodemix (a mixture of natural graphite having a volume-based averageparticle diameter (D50) of 15 μm, a styrene-1,3-butadiene copolymer, andsodium carboxymethylcellulose (at a weight ratio of 98:1:1)) wasdisposed on one surface of a negative electrode current collector(copper foil). To the negative electrode plate, a pressure of 40 MPa wasapplied at a room temperature with use of a roll pressing machine. Anegative electrode plate A was thus obtained.

The positive electrode plate A obtained and the negative electrode plateA obtained were each subjected to the folding endurance test (describedearlier). As a result, a positive electrode active material layer of thepositive electrode plate A was not peeled from the positive electrodeplate A until the positive electrode plate A was bent 64 times, and anegative electrode active material layer of the negative electrode plateA was not peeled from the negative electrode plate A until the negativeelectrode plate A was bent 1325 times.

That is, it was revealed that application of an excessive pressureduring production of an electrode plate may make it impossible to obtainan electrode plate that satisfies the above-described requirements ofthe number of times of bending of the electrode plate.

INDUSTRIAL APPLICABILITY

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention is excellent in charge capacitycharacteristic after being subjected to a charge and discharge cycle.Thus, the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can be suitably used as a batteryfor, for example, any of a personal computer, a mobile telephone, aportable information terminal, and a vehicle.

The invention claimed is:
 1. A nonaqueous electrolyte secondary battery comprising: a nonaqueous electrolyte secondary battery separator including a polyolefin porous film; a porous layer containing a polyvinylidene fluoride-based resin; a positive electrode plate comprising a positive electrode active material layer, wherein the positive electrode active material layer is not peeled from the positive electrode plate until the positive electrode plate is bent 130 or more times in a folding endurance test carried out in conformity with an MIT tester method defined in JIS P 8115 (1994), under a load of 1 N, and at a bending angle of 45°; and a negative electrode plate comprising a negative electrode active material layer, wherein the negative electrode active material layer is not peeled from the negative electrode plate until the negative electrode plate is bent 1650 or more times in the folding endurance test, wherein: when diethyl carbonate is dropped on the polyolefin porous film, the diethyl carbonate dropped on the polyolefin porous film diminishes at a rate of 15 sec/mg to 21 sec/mg; the diethyl carbonate has a spot diameter of not less than 20 mm 10 seconds after the diethyl carbonate was dropped on the polyolefin porous film; the porous layer is provided between the nonaqueous electrolyte secondary battery separator and at least one of the positive electrode plate and the negative electrode plate; and the polyvinylidene fluoride-based resin contained in the porous layer contains an α-form polyvinylidene fluoride-based resin in an amount of not less than 35.0 mol % with respect to 100 mol % of a total amount of the α-form polyvinylidene fluoride-based resin and a β-form polyvinylidene fluoride-based resin contained in the polyvinylidene fluoride-based resin, where the amount of the α-form polyvinylidene fluoride-based resin contained is calculated from waveform separation of (α/2) observed at around −78 ppm in a ¹⁹F-NMR spectrum obtained from the porous layer, and waveform separation of {(α/2)+β} observed at around −95 ppm in the ¹⁹F-MMR spectrum obtained from the porous layer.
 2. The nonaqueous electrolyte secondary battery as set forth in claim 1, wherein the positive electrode active material layer contains a transition metal oxide.
 3. The nonaqueous electrolyte secondary battery as set forth in claim 1, wherein the negative electrode active material layer contains graphite.
 4. The nonaqueous electrolyte secondary battery as set forth in claim 1, further comprising: another porous layer which is provided between (i) the nonaqueous electrolyte secondary battery separator and (ii) at least one of the positive electrode plate and the negative electrode plate.
 5. The nonaqueous electrolyte secondary battery as set forth in claim 4, wherein the another porous layer contains at least one resin selected from the group consisting of a polyolefin, a (meth)acrylate-based resin, a fluorine-containing resin excluding a polyvinylidene fluoride-based resin, a polyamide-based resin, a polyester-based resin, and a water-soluble polymer.
 6. The nonaqueous electrolyte secondary battery as set forth in claim 5, wherein the polyamide-based resin is an aramid resin. 